Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally include one or more active layers of semiconductor material sandwiched between oppositely doped layers. When bias is applied across doped layers, holes and electrons are injected into one or more active layers where they recombine to generate light that is emitted from the device.
Residential and commercial lighting are increasingly utilizing LED lighting in place of incandescent and fluorescent lighting. In a common arrangement, LED light boards are made with connections (bases) that can be used as a direct replacement for an incandescent bulb. LED boards also require driver circuitry to operate the board. In cases where an LED board is intended as a direct replacement for an incandescent bulb, the LED board must include the driver circuitry as part of the board because such circuitry is not already associated with the fixture.
Conventional AC direct drive LEDs use a fixed length LED string driven by a constant current regulator (CCR). The CCR is an electronic circuit that regulates the current running through it independent of the voltage applied This arrangement operates by rectifying an AC input. After the AC input is rectified, the resulting DC output is a half pulse sinusoidal voltage curve at 120 Hz. Within each cycle, the LEDs will turn on when the input voltage reaches the turn on voltage of the LED string and work at full load within a very short time until the voltage drops below the turn on voltage of the LED string. The transient time of this circuit is relatively small and can be neglected.
The CCR will begin to limit output current when the working current of the LED string reaches the anticipated current level. While the given circuit input current remains the same from this time point on, the power consumption of the LED string is a constant represented by the formula:PLED=Vforward×Iregulate  (1)
The input voltage will vary over the whole cycle which means the input power is:Pinput=Vline×Iregulate  (2)
Subtracting (1) from (2) yields the power dissipated by the CCR. The relative power used to illuminate the LEDs and power lost in the CCR are displayed graphically in FIG. 1. With respect to the single 120 Hz half-input wave shown, the horizontal scale of the rectangular box illustrates the time period the LED is illuminated during that half-wave input and its area represents the power used to illuminate the LED. The areas under the curve, but outside the rectangular box represents the power that is wasted in the driver circuitry.
The optimization of the efficiency in the conventional circuit is a tradeoff between the light output duty cycle and the wasted power consumed by the CCR. That is, as LEDs with higher or lower turn-on voltages are selected, different on times for the LED can be obtained, but the amount of power is wasted in the driver circuit is also affected.
It would be desirable to provide a LED board that utilizes more of the wasted energy in the driver circuit. This would provide a board that produces more light for a given number of LEDs or produces a given light output with fewer LEDs. It would also reduce the heat created by the board and improve board energy efficiency, which would further allow the board to utilize smaller heat sinks, or eliminate the heat sink entirely.